A temperature gradient can be induced in an optical fiber containing a fiber Bragg grating (FBG) in order to change the characteristic spectral response of the grating. Such thermally adjustable devices are very useful for optical communication systems as well as other applications. Various techniques are known in the art to impose a temperature change or gradient to a FBG for various purposes. For example, uniform heating along the length of the grating allows shifting the spectral response of the device, while a variable heating along said length allows adjusting the bandwidth and/or dispersion of the grating.
U.S. Pat. No. 6,842,567 (“the '567 patent”), issued on Jan. 11, 2005, shows examples of assemblies useful for applying a temperature gradient to a FBG. The temperature gradient is produced in a heat conductive element, with which the FBG is in continuous thermal contact, and heat pumping devices controlling the temperature of the ends of the heat conductive element. A first preferred embodiment includes a heat recirculation member allowing the recirculation of heat between the two ends of the heat conductive elongated element, thereby providing a rapid and dynamical tuning of the temperature gradient with a minimal heat loss. A second embodiment provides isolation from the surrounding environment in order to decouple the desired temperature gradient from ambient temperature fluctuations, thereby improving the control of the optical response of a fiber grating. This isolation can for example be provided by inserting the elongated element in a vacuum chamber within an insulating enclosure.
In many applications, the thermal gradient applied to the grating should ideally be linear. In principle, a linear temperature gradient can be created between the ends of an elongated element, such as in the '567 patent, for example, if the ends are maintained at different temperatures and if heat transport takes place only between these ends. In practice, heat loss from the elongated element to the surroundings distorts the thermal gradient whose profile therefore diverges from the theoretical linear form. In order to improve the linearity of the thermal gradient along the conductive elongated element, these heat loss mechanisms between the elongated element and the surroundings should be minimized.
As the low emissivity of a metallic elongated element reduces radiative losses, the heat loss in typical temperature gradient assemblies mainly stems from conduction in the air, since the distance between the two elements is usually minimized to avoid convection. FIG. 1A (Prior Art) illustrates the effect of conductive heat loss on the temperature gradient along the elongated element when the average of the temperature at both ends of the elongated element is than warmer than the surroundings (Tc>T∞). The heat loss is seen to distort the thermal gradient, the temperature distortion being indicated as δT in the graph. FIG. 1B (Prior Art) illustrates the effect of convective heat loss on the temperature gradient along the elongated element when the average of the temperature at both ends is colder than the surroundings (Tc<T∞. FIG. 1C (Prior Art) illustrates the effective temperature gradient along the elongated element when the environment temperature is equal to the average temperature between both extremities (T∞=(T1+T2)/2=Tc, as is the case when the grating is secured in an insulating enclosure, as taught by the '567 patent.
As seen from FIG. 1C, in order to minimize discrepancies, temperature in the immediate surroundings of the elongated element can be fixed at the midpoint value between the two extremities (Tc=(T1+T2)/2), but there still remains an inversed S-shape temperature profile. This has the effect of lowering the magnitude of the effective temperature profile slope in the middle of the useful bandwidth, where the slope can be from 20 to 30% smaller than the imposed gradient, as seen in FIG. 1C. Larger gradients are therefore required in order to reach the originally intended temperature slope. In order to correct for this effect, the group delay profile of the gratings can be overcompensated in the opposite direction, but such an approach is a tradeoff intended for correcting the highest dispersion settings (i.e. at the strongest gradient), and results in imperfect matches for smaller dispersion settings. Moreover, in order to cover the whole originally intended tuning range, larger gradients must be used, which is not always possible, and at best requires operation at higher temperatures and increased power consumption.
In view of the above, there is therefore a need for an improved assembly for applying a thermal gradient to a FBG or the like.